U.S. patent application number 12/791979 was filed with the patent office on 2010-12-09 for dressing apparatus, dressing method, and polishing apparatus.
Invention is credited to Hiroyuki SHINOZAKI.
Application Number | 20100311309 12/791979 |
Document ID | / |
Family ID | 43261108 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100311309 |
Kind Code |
A1 |
SHINOZAKI; Hiroyuki |
December 9, 2010 |
DRESSING APPARATUS, DRESSING METHOD, AND POLISHING APPARATUS
Abstract
A dressing apparatus for use in a polishing apparatus for
polishing a substrate to planarize a surface of the substrate is
disclosed. The dressing apparatus includes a dresser disk, a
dresser drive shaft coupled to the dresser disk, a pneumatic
cylinder configured to press the dresser disk against the polishing
pad through the dresser drive shaft, a pressure-measuring device
configured to measure pressure of the gas supplied to the pneumatic
cylinder, a load-measuring device configured to measure a load
acting on the dresser drive shaft, and a pressure controller
configured to control the pressure of the gas supplied to the
pneumatic cylinder. The pressure controller is configured to
establish a relationship between the pressure of the gas and a
pressing force of the dresser disk against the polishing pad, based
on measurement values of the pressure-measuring device and the
load-measuring device.
Inventors: |
SHINOZAKI; Hiroyuki; (Tokyo,
JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
1030 15th Street, N.W.,, Suite 400 East
Washington
DC
20005-1503
US
|
Family ID: |
43261108 |
Appl. No.: |
12/791979 |
Filed: |
June 2, 2010 |
Current U.S.
Class: |
451/5 ; 451/294;
451/443; 451/56 |
Current CPC
Class: |
B24B 49/08 20130101;
B24B 53/017 20130101; B24B 49/16 20130101 |
Class at
Publication: |
451/5 ; 451/443;
451/294; 451/56 |
International
Class: |
B24B 49/00 20060101
B24B049/00; B24B 55/00 20060101 B24B055/00; B24B 5/00 20060101
B24B005/00; B24B 1/00 20060101 B24B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 4, 2009 |
JP |
2009-134914 |
Claims
1. A dressing apparatus for dressing a polishing pad, said
apparatus comprising: a dresser disk to be brought into sliding
contact with the polishing pad; a vertically movable dresser drive
shaft coupled to said dresser disk; a pressing mechanism configured
to receive supply of a gas to press said dresser disk against the
polishing pad through said dresser drive shaft; a
pressure-measuring device configured to measure pressure of the gas
supplied to said pressing mechanism; a load-measuring device
configured to measure a load acting on said dresser drive shaft;
and a pressure controller configured to control the pressure of the
gas supplied to said pressing mechanism, wherein said pressure
controller is configured to establish a relationship between the
pressure of the gas and a pressing force of said dresser disk
against the polishing pad, based on measurement values of said
pressure-measuring device and said load-measuring device.
2. A polishing apparatus for polishing a substrate, said apparatus
comprising: a rotatable polishing table for supporting a polishing
pad; a top ring configured to press the substrate against the
polishing pad; and a dressing apparatus according to claim 1.
3. A dressing apparatus for dressing a polishing pad, said
apparatus comprising: a dresser disk to be brought into sliding
contact with the polishing pad; a vertically movable dresser drive
shaft coupled to said dresser disk; a pneumatic cylinder configured
to press said dresser disk against the polishing pad through said
dresser drive shaft; a lifting mechanism configured to lift said
dresser disk through said dresser drive shaft; and a pressure
controller configured to control pressure of a gas supplied to said
pneumatic cylinder.
4. The dressing apparatus according to claim 3, wherein said
lifting mechanism comprises a spring.
5. The dressing apparatus according to claim 3, further comprising:
a position sensor configured to measure a position of said dresser
disk in a vertical direction when said dresser disk is in contact
with the polishing pad.
6. The dressing apparatus according to claim 5, wherein said
pressure controller is configured to change the pressure of the gas
supplied to said pneumatic cylinder based on a measurement value of
said position sensor.
7. The dressing apparatus according to claim 5, further comprising:
a load-measuring device configured to measure a load acting on said
dresser drive shaft; and a pressure-measuring device configured to
measure the pressure of the gas supplied to said pneumatic
cylinder, wherein said pressure controller is configured to
determine an amount of wear of the polishing pad from a measurement
value of said position sensor and establish a relationship between
the pressure of the gas and a pressing force of said dresser disk
against the polishing pad, based on measurement values of said
pressure-measuring device and said load-measuring device, when the
amount of wear of the polishing pad has reached a predetermined
value.
8. The dressing apparatus according to claim 3, further comprising:
a load-measuring device configured to measure a load acting on said
dresser drive shaft; and a pressure-measuring device configured to
measure the pressure of the gas supplied to said pneumatic
cylinder, wherein said pressure controller is configured to
establish a relationship between the pressure of the gas and a
pressing force of said dresser disk against the polishing pad,
based on measurement values of said pressure-measuring device and
said load-measuring device.
9. The dressing apparatus according to claim 3, further comprising:
a load-measuring device configured to measure a load acting on said
dresser drive shaft, wherein said pressure controller is configured
to control the pressure of the gas based on a measurement value of
said load-measuring device such that a pressing force of said
dresser disk against the polishing pad is kept at a predetermined
target value during dressing of the polishing pad.
10. A polishing apparatus for polishing a substrate, said apparatus
comprising: a rotatable polishing table for supporting a polishing
pad; a top ring configured to press the substrate against the
polishing pad; and a dressing apparatus according to claim 3.
11. A method of dressing a polishing pad, said method comprising:
rotating a dresser disk and the polishing pad; pressing the dresser
disk against the polishing pad through a dresser drive shaft by a
pressing mechanism that is actuated by receiving supply of a gas;
measuring pressure of the gas supplied to the pressing mechanism;
measuring a load acting on the dresser drive shaft; and
establishing a relationship between the pressure of the gas and a
pressing force of said dresser disk against the polishing pad,
based on measurement values of the pressure of the gas and
measurement values of the load.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a dressing apparatus and a
dressing method for dressing a polishing pad used in polishing of a
substrate, such as a semiconductor wafer. More particularly, the
present invention relates to a dressing apparatus and a dressing
method used in a polishing apparatus for polishing the substrate to
planarize a surface of the substrate. The present invention also
relates to a polishing apparatus having such a dressing
apparatus.
[0003] 2. Description of the Related Art
[0004] Semiconductor devices become smaller and smaller in recent
years, and device structures become more complicated. A surface
planarization is an essential process in fabrication of the
semiconductor devices. A typical technique used in the surface
planarization is chemical mechanical polishing (CMP). In this
chemical mechanical polishing, a substrate is brought into sliding
contact with a polishing surface of a polishing pad, while a
polishing liquid, containing abrasive particles such as silica
(SiO.sub.2), is supplied onto the polishing surface, whereby a
surface of the substrate is polished.
[0005] The chemical mechanical polishing is performed using a CMP
apparatus. The CMP apparatus includes a polishing table with a
polishing pad attached to an upper surface thereof, and a top ring
for holding a substrate, such as a semiconductor wafer, which is a
workpiece to be polished. While the polishing table and the top
ring are rotated about their own axes respectively, the top ring
presses the substrate against a polishing surface (i.e., an upper
surface) of the polishing pad at predetermined pressure to cause
sliding contact between the substrate and the polishing pad. In
this state, the polishing liquid is supplied onto the polishing
surface of the polishing pad. The substrate is thus polished in the
presence of the polishing liquid between the substrate and the
polishing pad. The surface of the substrate is planarized by a
combination of a chemical polishing action by alkali and a
mechanical polishing action by abrasive particles.
[0006] When the substrate is polished, the abrasive particles and
polishing debris adhere to the polishing surface (the upper
surface) of the polishing pad. In addition, characteristics of the
polishing pad are altered and its polishing performance is lowered.
Consequently, as polishing of the substrate is repeated, a
polishing speed (i.e., a removal rate) is lowered and uneven
polishing occurs. Thus, in order to regenerate the deteriorated
polishing surface of the polishing pad, a dressing apparatus is
provided adjacent to the polishing table. This dressing apparatus
regenerates the polishing surface of the polishing pad by slightly
scraping off the polishing surface.
[0007] FIG. 1 is a schematic view showing a conventional dressing
apparatus. As shown in FIG. 1, the dressing apparatus includes a
dresser disk 131, an air cylinder 136 for pressing the dresser disk
131 against a polishing pad 10, and a dresser drive shaft 132
coupling the dresser disk 131 and the air cylinder 136 to each
other. The dresser drive shaft 132 is divided into a rotating
section coupled to the dresser disk 131 and a non-rotating section
coupled to the air cylinder 136. The rotating section and the
non-rotating section are coupled to each other via a coupling
137.
[0008] The rotating section of the dresser drive shaft 132 is
supported by a ball spline 135. This ball spline 135 is a linear
motion guide which transmits a torque to the dresser drive shaft
132, while allowing a straight line motion of the dresser drive
shaft 132 in a longitudinal direction thereof. The ball spline 135
is coupled to a motor (not shown), so that the dresser disk 131 is
rotated by the motor through the dresser drive shaft 132.
[0009] The air cylinder 136 is a double-acting air cylinder in
which two pressure chambers are provided on both sides of a piston
136a. Air, with adjusted pressure, is injected into each pressures
chamber. Specifically, compressed air to generate a load on the
polishing pad 10 is introduced into the upper pressure chamber, and
on the other hand compressed air to support a weight of a movable
section, including the dresser disk 131 and the dresser drive shaft
132, is introduced into the lower pressure chamber. The pressure of
the air supplied to the lower pressure chamber is kept constant. A
pressing force of the dresser disk 131 against the polishing pad 10
is determined by differential pressure between the upper pressure
chamber and the lower pressure chamber.
[0010] Hard abrasive particles, such as diamond particles, are
fixed to a lower surface of the dresser disk 131. This lower
surface of the dresser disk 131 constitutes a dressing surface for
conditioning the polishing surface of the polishing pad 10. When
dressing the polishing pad 10, the dresser disk 131 is pressed
against the polishing pad 10, while a polishing table 11 and the
dresser disk 131 are rotated and pure water is supplied onto the
polishing surface of the polishing pad 10. The polishing surface of
the polishing pad 10 is dressed (or conditioned) by sliding contact
between the dressing surface of the dresser disk 131 and the
polishing surface.
[0011] During dressing, the polishing surface of the polishing pad
10 is scraped by the dresser disk 131. The pressing force of the
dresser disk 131 against the polishing pad 10 has a great influence
on a life of the polishing pad 10. Therefore, it is necessary to
accurately control the pressing force of the dresser disk 131. In
the above-described structures, since the air having constant
pressure is supplied into the lower pressure chamber of the air
cylinder 136, the pressing force of the dresser disk 131 depends on
the pressure of the air introduced into the upper pressure chamber.
Thus, calibration is necessary in order to establish a relationship
between the pressing force of the dresser disk 131 and the pressure
of the air introduced into the upper pressure chamber of the air
cylinder 136.
[0012] The calibration is performed by inserting a load-measuring
device (e.g., a load cell) between the polishing pad 10 and the
dresser disk 131 and associating measurement value (i.e., the
pressing force), obtained from the load-measuring device, with the
pressure of the air supplied to the air cylinder 136. However, in
order to carry out the calibration, it is necessary to stop
operations of the polishing apparatus. As a result an operation
rate of the polishing apparatus is lowered.
[0013] In addition to the above-described problem, the dressing
apparatus using the air cylinder entails the following drawback. As
described above, the pressing force of the dresser disk 131 against
the polishing pad 10 affects the lifetime of the polishing pad 10.
Therefore, in order to extend the life of the polishing pad 10, it
is necessary to decrease the pressing force of the dresser disk 131
to some extent. However, when the pressure of the air in the upper
pressure chamber of the air cylinder 136 is lowered, the piston may
not move in spite of the differential pressure between the upper
pressure chamber and the lower pressure chamber. This is because,
when the differential pressure between the upper pressure chamber
and the lower pressure chamber is close to zero, frictional
resistance between the piston and a cylinder and frictional
resistance between the dresser drive shaft 132 and the air cylinder
136 become relatively high. In such a dead zone where the air
cylinder 136 does not operate, good dressing of the polishing pad
10 is not performed and as a result, stable polishing performance
of the polishing pad 10 cannot be achieved.
SUMMARY OF THE INVENTION
[0014] The present invention has been made in view of the
above-described drawbacks. It is therefore a first object of the
present invention to provide a dressing apparatus and a dressing
method capable of establishing a relationship between pressing
force of a dresser disk and pressure of a gas generating the
corresponding pressing force, without stopping operations of a
polishing apparatus.
[0015] It is a second object of the present invention to provide a
dressing apparatus capable of stably producing a low pressing force
of the dresser disk.
[0016] In order to achieve the above object, one aspect of the
present invention provides a dressing apparatus for dressing a
polishing pad. The apparatus includes: a dresser disk to be brought
into sliding contact with the polishing pad; a vertically movable
dresser drive shaft coupled to the dresser disk; a pressing
mechanism configured to receive supply of a gas to press the
dresser disk against the polishing pad through the dresser drive
shaft; a pressure-measuring device configured to measure pressure
of the gas supplied to the pressing mechanism; a load-measuring
device configured to measure a load acting on the dresser drive
shaft; and a pressure controller configured to control the pressure
of the gas supplied to the pressing mechanism. The pressure
controller is configured to establish a relationship between the
pressure of the gas and a pressing force of the dresser disk
against the polishing pad, based on measurement values of the
pressure-measuring device and the load-measuring device.
[0017] Another aspect of the present invention is to provide a
polishing apparatus for polishing a substrate. The apparatus
includes: a rotatable polishing table for supporting a polishing
pad; a top ring configured to press the substrate against the
polishing pad; and the above-described dressing apparatus.
[0018] Still another aspect of the present invention is to provide
a dressing apparatus for dressing a polishing pad. The apparatus
includes: a dresser disk to be brought into sliding contact with
the polishing pad; a vertically movable dresser drive shaft coupled
to the dresser disk; a pneumatic cylinder configured to press the
dresser disk against the polishing pad through the dresser drive
shaft; a lifting mechanism configured to lift the dresser disk
through the dresser drive shaft; and a pressure controller
configured to control pressure of a gas supplied to the pneumatic
cylinder.
[0019] In a preferred aspect of the present invention, the lifting
mechanism comprises a spring.
[0020] In a preferred aspect of the present invention, the dressing
apparatus further includes a position sensor configured to measure
a position of the dresser disk in a vertical direction when the
dresser disk is in contact with the polishing pad.
[0021] In a preferred aspect of the present invention, the pressure
controller is configured to change the pressure of the gas supplied
to the pneumatic cylinder based on a measurement value of the
position sensor.
[0022] In a preferred aspect of the present invention, the dressing
apparatus further includes: a load-measuring device configured to
measure a load acting on the dresser drive shaft; and a
pressure-measuring device configured to measure the pressure of the
gas supplied to the pneumatic cylinder. The pressure controller is
configured to determine an amount of wear of the polishing pad from
a measurement value of the position sensor and establish a
relationship between the pressure of the gas and a pressing force
of the dresser disk against the polishing pad, based on measurement
values of the pressure-measuring device and the load-measuring
device, when the amount of wear of the polishing pad has reached a
predetermined value.
[0023] In a preferred aspect of the present invention, the dressing
apparatus further includes: a load-measuring device configured to
measure a load acting on the dresser drive shaft; and a
pressure-measuring device configured to measure the pressure of the
gas supplied to the pneumatic cylinder. The pressure controller is
configured to establish a relationship between the pressure of the
gas and a pressing force of the dresser disk against the polishing
pad, based on measurement values of the pressure-measuring device
and the load-measuring device.
[0024] In a preferred aspect of the present invention, the dressing
apparatus further includes a load-measuring device configured to
measure a load acting on the dresser drive shaft. The pressure
controller is configured to control the pressure of the gas based
on a measurement value of the load-measuring device such that a
pressing force of the dresser disk against the polishing pad is
kept at a predetermined target value during dressing of the
polishing pad.
[0025] Still another aspect of the present invention is to provide
a polishing apparatus for polishing a substrate. The apparatus
includes: a rotatable polishing table for supporting a polishing
pad; a top ring configured to press the substrate against the
polishing pad; and the above-described dressing apparatus.
[0026] Still another aspect of the present invention is to provide
a method of dressing a polishing pad. The method includes: rotating
a dresser disk and the polishing pad; pressing the dresser disk
against the polishing pad through a dresser drive shaft by a
pressing mechanism that is actuated by receiving supply of a gas;
measuring pressure of the gas supplied to the pressing mechanism;
measuring a load acting on the dresser drive shaft; and
establishing a relationship between the pressure of the gas and a
pressing force of the dresser disk against the polishing pad, based
on measurement values of the pressure of the gas and measurement
values of the load.
[0027] According to the present invention, the load-measuring
device, incorporated in the dresser drive shaft, can establish the
relationship between the pressing force and the pressure of the gas
within a very short period of time before or after the dressing
operation or during the dressing operation. Therefore, it is not
necessary to stop the operations of the polishing apparatus and as
a result the operation rate of the polishing apparatus can be
improved.
[0028] Further, according to the present invention, providing of
the lifting mechanism enables setting of a large gas-pressure
difference between two pressure chambers in the air cylinder.
Therefore, operating zone of the air cylinder lies out of the dead
zone (which is a zone where the piston does not operate in spite of
a change in the differential pressure). Hence, the air cylinder can
generate low pressing forces stably.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic view showing a conventional dressing
apparatus;
[0030] FIG. 2 is a perspective view of a polishing apparatus;
[0031] FIG. 3 is a schematic view showing a dressing apparatus
according to a first embodiment of the present invention;
[0032] FIG. 4 is a graph showing a relationship, obtained by
calibration, between pressing force of a dresser disk and pressure
of air in an upper pressure chamber;
[0033] FIG. 5 is a schematic view showing a dressing apparatus
according to a second embodiment of the present invention;
[0034] FIG. 6 is a schematic view showing a modified example of the
dressing apparatus according to the second embodiment of the
present invention;
[0035] FIG. 7 is a graph showing a relationship between the
pressure of the air in the upper pressure chamber of the air
cylinder and the pressing force applied to a polishing pad;
[0036] FIG. 8 is a schematic view showing a dressing apparatus
according to a third embodiment of the present invention;
[0037] FIG. 9 is a schematic view showing a modified example of the
dressing apparatus according to the third embodiment of the present
invention;
[0038] FIG. 10 is a schematic view showing a dressing apparatus
according to a fourth embodiment of the present invention;
[0039] FIG. 11 is a schematic view showing a modified example of
the dressing apparatus according to the fourth embodiment of the
present invention;
[0040] FIG. 12 is a schematic view showing a dressing apparatus
according to a fifth embodiment of the present invention;
[0041] FIG. 13 is a schematic view showing a modified example of
the dressing apparatus according to the fifth embodiment of the
present invention; and
[0042] FIG. 14 is a schematic view showing a dressing apparatus
according to a sixth embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] Embodiments of the present invention will be described below
with reference to the drawings. Like or corresponding structural
elements are denoted by the same reference numerals in the
following description and repetitive descriptions thereof will be
omitted.
[0044] FIG. 2 is a perspective view showing a polishing apparatus.
The polishing apparatus includes a polishing table 11 supporting a
polishing pad 10, a top ring unit 20 for polishing a substrate
(i.e., a workpiece to be polished), such as a wafer, by bringing it
into sliding contact with the polishing pad 10, and a dressing unit
(dressing apparatus) 30 configured to condition (i.e., dress) an
upper surface of the polishing pad 10. The polishing pad 10 is
attached to an upper surface of the polishing table 11, and an
upper surface of the polishing pad 10 provides a polishing surface.
The polishing table 11 is coupled to a motor (not shown), so that
the polishing table 11 and the polishing pad 10 are rotated by the
motor in a direction indicated by arrow.
[0045] The top ring unit 20 includes a top ring 21 configured to
hold the substrate and press it against the upper surface of the
polishing pad 10, a top ring drive shaft 22 coupled to the top ring
21, and a top ring swing arm 23 rotatably holding the top ring
drive shaft 22. The top ring swing arm 23 is supported by a top
ring swing shaft 24. A motor (not shown) is installed in the top
ring swing arm 23 and this motor is coupled to the top ring drive
shaft 22. Rotation of this motor is transmitted to the top ring 21
via the top ring drive shaft 22, whereby the top ring 21 is rotated
about the top ring drive shaft 22 in a direction indicated by
arrow.
[0046] A liquid supply mechanism 25 for supplying a polishing
liquid and a dressing liquid onto the polishing surface of the
polishing pad 10 is provided adjacent to the top ring unit 20. This
liquid supply mechanism 25 has plural supply nozzles (not shown)
from which the polishing liquid and the dressing liquid are
supplied separately onto the polishing surface of the polishing pad
10. The liquid supply mechanism 25 serves as both a
polishing-liquid supply mechanism for supplying the polishing
liquid onto the polishing pad 10 and a dressing-liquid supply
mechanism for supplying the dressing liquid (e.g., pure water) onto
the polishing pad 10. The polishing-liquid supply mechanism and the
dressing-liquid supply mechanism may be provided separately.
[0047] The top ring 21 has a lower surface that provides a
substrate-holding surface for holding the substrate by a vacuum
suction or the like. The top ring drive shaft 22 is coupled to a
non-illustrated vertical-movement actuator (e.g., an air cylinder).
With this configuration, the top ring 21 is elevated and lowered by
the vertical-movement actuator through the top ring drive shaft 22.
The top ring swing shaft 24 is located radially outwardly of the
polishing table 11. This top ring swing shaft 24 is configured to
rotate, so that the top ring 21 can move between a polishing
position on the polishing pad 10 and a rest position outside the
polishing pad 10.
[0048] Polishing of the substrate is performed as follows. The
substrate is held on the lower surface of the top ring 21, and the
top ring 21 and the polishing table 11 are rotated. In this state,
the polishing liquid is supplied onto the polishing surface of the
polishing pad 10, and then the top ring 21 presses the substrate
against the polishing surface of the polishing pad 10. A surface (a
lower surface) of the substrate is polished by the mechanical
polishing action of abrasive particles contained in the polishing
liquid and the chemical polishing action of the polishing
liquid.
[0049] The dressing unit (dressing apparatus) 30 includes a dresser
disk 31 to be brought into sliding contact with the polishing
surface of the polishing pad 10, a dresser drive shaft 32 coupled
to the dresser disk 31, and a dresser swing arm 33 rotatably
holding the dresser drive shaft 32. A lower surface of the dresser
disk 31 provides a dressing surface that is brought into sliding
contact with the polishing surface of the polishing pad 10. Hard
abrasive particles, such as diamond particles, are fixed to the
dressing surface. The dresser swing arm 33 is supported by a
dresser swing shaft 34. A motor (not shown) is installed in the
dresser swing arm 33 and this motor is coupled to the dresser drive
shaft 32. Rotation of this motor is transmitted to the dresser disk
31 via the dresser drive shaft 32, whereby the dresser disk 31 is
rotated about the dresser drive shaft 32 in a direction indicated
by arrow.
[0050] The dresser swing shaft 34 is coupled to a swing motor (not
shown). When the swing motor is set in motion, the dresser disk 31
is moved on the polishing surface of the polishing pad 10 in
substantially a radial direction of the polishing surface. When
dressing the polishing pad 10, the dresser disk 31 is pressed
against the polishing pad 10, while the polishing table 11 and the
dresser disk 31 are rotated and the dressing liquid is supplied
onto the polishing surface of the polishing pad 10. The polishing
surface of the polishing pad 10 is conditioned by sliding contact
between the dressing surface of the dresser disk 31 and the
polishing surface. During dressing, the dresser disk 31 is
oscillated in the radial direction of the polishing pad.
[0051] FIG. 3 is a schematic view showing the dressing unit 30
according to a first embodiment of the present invention. As shown
in FIG. 3, the dressing unit 30 includes an air cylinder (a
pneumatic cylinder) 36 as a pressing mechanism for pressing the
dresser disk 31 against the polishing pad 10 through the dresser
drive shaft 32. The dresser drive shaft 32 is supported by a ball
spline 35. This ball spline 35 is a linear motion guide which
transmits a torque to the dresser drive shaft 32, while allowing a
straight line motion of the dresser drive shaft 32 in a
longitudinal direction thereof. The ball spline 35 is rotatably
supported by bearings 48, which are fixedly mounted on a support
base 49 secured to the dresser swing arm 33. Relative positions of
the support base 49 and the ball spline 35 in a vertical direction
with respect to the dresser swing arm 33 are fixed.
[0052] A motor (not shown) is coupled to the ball spline 35 and
this motor causes the dresser disk 31 to rotate through the dresser
drive shaft 32. The dresser drive shaft 32 is divided into a
rotating section coupled to the dresser disk 31 and a non-rotating
section coupled to the air cylinder 36. The rotating section and
the non-rotating section are coupled to each other by a coupling
37. The rotating section of the dresser drive shaft 32 has a shape
of spline shaft and is supported by the ball spline 35 that allows
the dresser drive shaft 32 to move vertically.
[0053] An upper end of the dresser drive shaft 32 is coupled to the
air cylinder (pressing mechanism) 36, which is configured to press
the dresser disk 31 against the polishing pad 10 through the
dresser drive shaft 32. The air cylinder 36 is a double-acting air
cylinder in which two pressure chambers are disposed on both sides
of a piston 36a. This air cylinder 36 is a type of pneumatic
actuator. An electropneumatic regulator 40, serving as a
pressure-adjusting device, is coupled to an upper pressure chamber
of the air cylinder 36. This electropneumatic regulator 40 is
configured to adjust pressure of compressed air supplied from an
air source (not shown) and deliver the air of adjusted pressure Pc
to the upper pressure chamber of the air cylinder 36. Similarly, an
electropneumatic regulator 41, serving as a pressure-adjusting
device, is coupled to a lower pressure chamber of the air cylinder
36. The electropneumatic regulator 41 is configured to adjust
pressure of the compressed air supplied from the above-mentioned
air source and supply the air of adjusted pressure Pb to the lower
pressure chamber of the air cylinder 36. Instead of the air, other
type of gas may be used.
[0054] The air supplied to the upper pressure chamber generates a
load on the polishing pad 10, and on the other hand the air
supplied to the lower pressure chamber is counter air (or balance
air) for supporting a weight of vertically movable components
(which will be hereinafter referred to as "a dresser assembly")
which include the dresser disk 31 and the dresser drive shaft 32.
Pressure of the counter air is set to be large enough to support
the weight of the dresser assembly and is kept constant during
dressing. A pressing force of the dresser disk 31 against the
polishing pad 10 is determined by differential pressure between the
upper pressure chamber and the lower pressure chamber.
[0055] A load cell 45, which is a load-measuring device for
indirectly measuring the pressing force applied to the polishing
pad from the dresser disk 31, is provided in the dresser drive
shaft 32. The load cell 45 is coupled to a pressure controller 47
via an amplifier 46. Measurement values (output signals) of the
load cell 45 are amplified by the amplifier 46, and the amplified
measurement values are transmitted to the pressure controller
47.
[0056] The pressing force acting on the polishing pad 10 is a
resultant force of a downward force generated by the air cylinder
36 and the weight of the dresser assembly. More technically, the
pressing force acting on the polishing pad 10 is further affected
by frictional resistance between the ball spline 35 and the dresser
drive shaft 32 and frictional resistance in a sealing element of
the air cylinder 36. However, these frictional resistances are
relatively minute compared to the force generated by the air
cylinder 36 and the weight of the dresser assembly. Therefore,
these frictional resistances will be omitted in below-described
explanations.
[0057] The load cell 45 is incorporated in the dresser drive shaft
32 and measures a load acting on the dresser drive shaft 32.
Therefore, there is a difference between the measurement value
obtained by the load cell 45 and the actual pressing force. A
pressing force F applied to the polishing pad 10 by the dresser
disk 31, a measurement value F' of the load cell 45, and a
difference between the pressing force F and the measurement value
F' will be described with reference to FIG. 3. In a structure shown
in FIG. 3, the pressing force F when the dresser disk 31 is in
contact with the polishing pad 10 is expressed as
F=Fc-Fb+m.sub.1g+m.sub.2g (1)
where Fc represents a downward force generated by the air of
pressure Pc introduced in the upper pressure chamber of the air
cylinder 36, Fb represents an upward force generated by the air of
pressure Pb introduced in the lower pressure chamber of the air
cylinder 36, m.sub.1g represents a weight of an upper part of the
dresser assembly with respect to the load cell 45 as the center of
the dresser assembly, and m.sub.2g represents a weight of a lower
part of the dresser assembly with respect to the load cell 45 as
the center of the dresser assembly.
[0058] The load cell 45 is configured to measure not only a
compressive force acting on the dresser drive shaft 32, but also a
tensile force. When the dresser disk 31 is out of contact with the
polishing pad 10, only the weight m.sub.2g of the lower part of the
dresser assembly acts as a tensile force on the load cell 45.
Therefore, the measurement value outputted from the load cell 45 in
this state is--m.sub.2g. On the other hand, when the dresser disk
31 is in contact with the polishing pad 10, the weight m.sub.2g of
the lower part of the dresser assembly is not exerted on the load
cell 45. The measurement value F' outputted from the load cell 45
when the dresser disk 31 is in contact with the polishing pad 10 is
expressed as
F'=Fc-Fb+m.sub.1g (2)
[0059] From the above equation (1) and the equation (2), the
difference .DELTA.S between the pressing force F and the
measurement value F' is given by
.DELTA.S=F-F'=m.sub.2g (3)
[0060] Therefore, the actual pressing force F can be determined by
adding the difference .DELTA.S (=m.sub.2g), as an amount of
correction, to the measurement value F' obtained from the load cell
45. This amount of correction .DELTA.S can be given by a
measurement value outputted from the load cell 45 when the dresser
disk 31 is out of contact with the polishing pad 10. Alternatively,
the amount of correction .DELTA.S may be determined by placing a
calibration load cell between the dresser disk 31 and the polishing
pad 10 and subtracting the measurement value F' of the load cell 45
from the actual pressing force of the dresser disk 31 applied to
the polishing pad 10 (i.e., the measurement value of the
calibration load cell). This amount of correction .DELTA.S
(=m.sub.2g) depends only on the weight of the lower part of the
dresser assembly and the value of .DELTA.S is substantially
constant. Therefore, once the amount of correction .DELTA.S is
obtained, the value thereof can be used as it is repetitively.
[0061] The operation of obtaining the amount of correction is
performed prior to processing of a substrate, and the amount of
correction obtained is stored in the pressure controller 47. This
pressure controller 47 adds the amount of correction m.sub.2g to
the measurement value F', transmitted from the load cell 45, to
thereby determine the pressing force F of the dresser disk 31
against the polishing pad 10.
[0062] The pressure controller 47 is configured to perform
calibration for establishing a relationship between the pressing
force F obtained from the measurement value F' of the load cell 45
and the pressure Pc of the air in the upper pressure chamber of the
air cylinder 36. A pressure sensor (pressure-measuring device) 42
for measuring the pressure Pc of the air supplied into the upper
pressure chamber of the air cylinder 36 is provided in the
electropneumatic regulator 40. A measurement value of the pressure
sensor 42 is transmitted to the pressure controller 47. The
pressure controller 47 associates the pressing force F with the
measurement value obtained at the same point of time by the
pressure sensor 42 to thereby establish the relationship between
the pressing force F of the dresser disk 31 and the air pressure Pc
in the upper pressure chamber.
[0063] According to the present embodiment, unlike a conventional
calibration, it is not necessary to stop the operations of the
polishing apparatus for the calibration. Further, it is not
necessary to sandwich a load-measuring device for calibration
between the dresser disk 31 and the polishing pad 10. Therefore,
calibration can be performed within a very short time and operation
rate of the polishing apparatus can be improved.
[0064] FIG. 4 is a graph showing the relationship, obtained by the
calibration, between the pressing force of the dresser disk 31 and
the pressure of the air in the upper pressure chamber. In FIG. 4, a
vertical axis indicates the pressing force F of the dresser disk 31
and a horizontal axis indicates the pressure Pc of the air in the
upper pressure chamber. As can be seen from the graph in FIG. 4,
the pressing force of the dresser disk 31 is approximately in
proportion to the pressure of the air in the upper pressure
chamber. Therefore, the air pressure for generating a desired
pressing force can be determined from the graph shown in FIG.
4.
[0065] The pressure controller 47 determines the air pressure
corresponding to a desired pressing force that is inputted through
an input device (now shown), based on the relationship, obtained by
the calibration, between the pressing force and the air pressure,
and commands the electropneumatic regulator 40 to supply the air
having the determined pressure to the upper pressure chamber of the
air cylinder 36. The air cylinder 36 imparts the pressing force to
the dresser disk 31, and the dresser disk 31 presses the polishing
pad 10 at the desired pressing force.
[0066] FIG. 5 is a schematic view showing the dressing unit
according to a second embodiment of the present invention.
Structures and operations of this embodiment, which will not be
described below, are identical to those of the above-described
first embodiment, and repetitive descriptions thereof will be
omitted. As shown in FIG. 5, the lower pressure chamber of the air
cylinder (i.e., pressing mechanism) 36 is vented to the atmosphere,
while the upper pressure chamber is provided with the compressed
air through the electropneumatic regulator 40, as with the
above-described first embodiment. The dressing unit according the
second embodiment includes a spring 50 for supporting the weight of
the dresser assembly including the dresser disk 31 and the dresser
drive shaft 32. This spring 50 is a lifting mechanism provided
separately from the air cylinder 36. In this embodiment, the load
cell 45 is not provided.
[0067] The spring 50 is mounted on a support base 52 that is
secured to the dresser swing arm 33. The spring 50 has an upper end
that is in contact with a spring stopper 51 secured to the dresser
drive shaft 32. With these arrangements, the spring 50 exerts a
force on the dresser drive shaft 32 in a direction opposite to the
direction in which the air cylinder 36 presses the dresser disk 31,
thereby biasing the dresser disk 31 upwardly through the dresser
drive shaft 32. The coupling 37, which serves to couple the
rotating section and the non-rotating section of the dresser drive
shaft 32 to each other, is located below the spring stopper 51. The
support base 52 for supporting the spring 50 and the support base
49 for supporting the ball spline 35 may be a single member.
[0068] FIG. 6 is a schematic view showing a modified example of the
dressing unit according to the second embodiment of the present
invention. In this modified example, the spring 50 is located below
the coupling 37. The spring stopper 51 is secured to the rotating
section of the dresser drive shaft 32. A lower end of the spring 50
is secured to the ball spline 35. The spring 50, the ball spline
35, and the dresser drive shaft 32 are rotated in unison.
[0069] In the dressing unit shown in FIG. 5 and FIG. 6, the
pressing force F of the dresser disk 31 against the polishing pad
10 is expressed as a resultant force of a downward force Fc[N]
generated by the air cylinder 36, a weight mg[N] of the dresser
assembly in its entirety, and an upward force Fb[N] generated by
the spring 50. FIG. 7 is a graph showing a relationship between the
pressure Pc of the air supplied to the upper pressure chamber of
the air cylinder 36 and the pressing force F acting on the
polishing pad 10. In FIG. 7, a vertical axis indicates the pressing
force F acting on the polishing pad 10, and a horizontal axis
indicates the pressure Pc of the air in the upper pressure chamber
of the air cylinder 36. A sign "+" along the vertical axis
indicates an upward force and a sign "-" indicates a downward
force. The graph shown in FIG. 7 is depicted on the assumption that
the dresser disk 31 is in contact with the polishing pad 10 and
that a length of the spring 50 is kept constant.
[0070] As shown in FIG. 7, when the pressure Pc is equal to or
greater than P1, the pressing force is applied to the polishing pad
10 from the dresser disk 31. Since the upward force Fb produced by
the spring 50 is added to the downward force Fc produced by the air
cylinder 36, the force Fc is greater than the pressing force F
acting on the polishing pad 10. The fact that the force Fc is large
means that there is a large difference in the air pressure between
the upper pressure chamber and the lower pressure chamber of the
air cylinder 36. That is, a dead zone of the air cylinder 36 (i.e.,
a pressure range in which the piston 36a does not move due to
frictional resistance between the piston 36a and a cylinder when
the difference in the air pressure between the upper pressure
chamber and the lower pressure chamber is small) is separated from
an operation range of the air cylinder 36. Therefore, even when the
pressing force F is small (e.g., 10 N or less), the air cylinder 36
can operate smoothly. Moreover, because the pressing force F can be
set small, an amount of the polishing pad 10 that is scraped off
can be small. Consequently, the life of the polishing pad 10 can be
increased.
[0071] The spring 50, as the lifting mechanism, does not have
sliding elements, unlike the air cylinder 36. Therefore, use of the
spring 50 does not cause an increase in the sliding resistance, and
the air cylinder 36 can change the pressing force F of the dresser
disk 31 smoothly within a wide range including 0[N]. As a result,
the dresser disk 31 can press the polishing pad 10 at small
pressing force F stably.
[0072] A coil spring is preferably used as the spring 50. Instead
of the spring 50, an air spring (e.g., an air bag formed by a
flexible or deformable material) with a gas enclosed therein may be
used as the lifting mechanism. In order to reduce the sliding
resistance, it is preferable to use, as the air cylinder 36, a
metal air cylinder which does not use a lip seal between a piston
and a cylinder or a non-contact seal air cylinder having a
non-contact seal disposed between a piston and a cylinder.
[0073] FIG. 8 is a schematic view showing the dressing unit
according to a third embodiment of the present invention.
Structures and operations of this embodiment, which will not be
described below, are identical to those of the above-described
second embodiment, and repetitive descriptions thereof will be
omitted. In this embodiment, load cell 45, which serves as a
load-measuring device, is integrated in the dresser drive shaft 32.
This load cell 45 is located between the air cylinder 36 and the
spring 50 and is electrically connected to the pressure controller
47 via the amplifier 46.
[0074] In this embodiment, a difference between the actual pressing
force of the dresser disk 31 and the measurement value of the load
cell 45 corresponds to the upward force of the spring 50 and the
weight of the dresser assembly. The difference between the pressing
force F of the dresser disk 31 and the measurement value F' of the
load cell 45 will be described below.
[0075] When the air is not supplied into the upper pressure chamber
of the air cylinder 36 (i.e., when Fc is zero), the dresser
assembly is elevated by the spring 50 and the dresser disk 31 is
located away from the polishing pad 10. Hereinafter, this state
will be referred to as an initial state. In this initial state, the
piston 36a is in contact with the upper end of the cylinder by
receiving the lifting force of the spring 50. This lifting force Fb
of the spring 50 is expressed as
Fb=Fb.sub.0+kZ (4)
where Fb.sub.0 is the lifting force [N] of the spring 50 in the
initial state, k is a spring constant [N/mm], and Z is a
displacement [mm] of the dresser assembly from its initial position
(i.e., a position in the initial state).
[0076] In the initial state, the force Fc of the air cylinder 36 is
zero. The displacement Z is also zero. Therefore, the lifting force
Fb of the spring 50 is Fb.sub.0. In the initial state, the weights
m.sub.1g and m.sub.2g of the dresser assembly and the lifting force
Fb.sub.0 (=Fb) of the spring 50 act on the load cell 45. The weight
m.sub.2g of the lower part of the dresser assembly acts as a
tensile force on the load cell 45. Therefore, the measurement value
F' of the load cell 45 is expressed by the following equation.
F'=Fb.sub.0+m.sub.1g-m.sub.2g (5)
[0077] When the air is supplied into the upper pressure chamber of
the air cylinder 36, it generates the downward force Fc. When the
downward force Fc exceeds a certain value, the dresser assembly is
lowered against the lifting force of the spring 50. When the
dresser assembly is lowered slightly from the initial position and
is still hanging in the air (i.e., Fc.noteq.0, Z.noteq.0, F=0), the
following equation holds from the condition of equilibrium of
forces.
F = Fc - Fb + m 1 g + m 2 g = Fc - ( Fb 0 + k Z ) + m 1 g + m 2 g =
0 ( 6 ) ##EQU00001##
[0078] Since the above equation (6) contains a variable Z, the
dresser assembly comes to rest at a certain position that depends
on the force Fc. Therefore, even if the pressing force F of the
dresser disk 31 is zero or approximately zero, the position of the
dresser disk 31 is stable. This indicates that the dresser disk 31
can dress the polishing pad 10 at a very small force.
[0079] The measurement value F' of the load cell 45 when the
dresser assembly is suspended is given by
F ' = Fc + Fb + m 1 g - m 2 g = Fc + ( Fb 0 + k Z ) + m 1 g - m 2 g
( 7 ) ##EQU00002##
[0080] As the force Fc is further increased, the dresser disk 31 is
further lowered to contact the polishing pad 10. In this contact
state (i.e., Fc.noteq.0, Z.noteq.0, F.noteq.0), the pressing force
F is expressed as follows.
F = Fc - Fb + m 1 g + m 2 g = Fc - ( Fb 0 + k Z ) + m 1 g + m 2 g (
8 ) ##EQU00003##
[0081] On the other hand, the measurement value F', as the output
of the load cell 45, is expressed as follows.
F ' = Fc + Fb + m 1 g - m 2 g = Fc + ( Fb 0 + k Z ) + m 1 g - m 2 g
( 9 ) ##EQU00004##
[0082] Accordingly, the difference .DELTA.S between the pressing
force F and the measurement value F' is given as follows.
.DELTA.S=F-F'=2m.sub.2g-2(Fb.sub.0+kZ) (10)
[0083] Therefore, the pressing force F can be given by adding the
difference .DELTA.S (=2m.sub.2g-2(Fb.sub.0+kZ)), as the amount of
correction, to the measurement value F' that is obtained from the
load cell 45. This amount of correction .DELTA.S can be given by
the known values Fb.sub.0, k, m.sub.2g and an actually measured
value of the displacement Z. Alternatively, a load cell for
calibration may be placed between the dresser disk 31 and the
polishing pad 10 to obtain an actual pressing force of the dresser
disk 31 applied to the polishing pad 10, and the amount of
correction .DELTA.S may be determined by subtracting the
measurement value F' of the load cell 45 from the actual pressing
force (i.e., a measurement value of the load cell for
calibration).
[0084] The amount of correction .DELTA.S is affected by the spring
constant k [N/mm] of the spring 50. More specifically, a position
of the dresser disk 31 in the vertical direction when the dresser
disk 31 is in contact with the polishing pad 10 (hereinafter, this
position will be referred to as a pressing position) is lowered in
accordance with wear of the polishing pad 10. When the pressing
position of the dresser disk 31 is lowered by AZ due to the wear of
the polishing pad 10, the lifting force Fb of the spring 50 is
increased by k.DELTA.Z. As a result, the pressing force F of the
dresser disk 31 is decreased by k.DELTA.Z. Therefore, use of the
spring having a small spring constant k can reduce the influence on
the pressing force F. For example, when the spring constant k is 1
N/mm and the amount of the wear of the polishing pad is 0.5 mm, the
pressing force F is decreased by about 0.5 N.
[0085] As with the first embodiment, the pressure controller 47
performs the calibration for determining the relationship between
the pressing force of the dresser disk 31 and the pressure of the
air supplied to the upper pressure chamber of the air cylinder 36,
based on the measurement values obtained from the load cell 45 and
the measurement values obtained from the pressure sensor 42. This
calibration is performed automatically by the pressure controller
47 at a predetermined timing, e.g., immediately before or
immediately after dressing of the polishing pad 10. The calibration
may be performed during dressing. Since the pressing force F varies
in accordance with the wear of the polishing pad 10 as described
above, it is preferable to carry out the calibration
periodically.
[0086] FIG. 9 is a schematic view showing a modified example of the
dressing unit according to the third embodiment of the present
invention. In this modified example, the spring 50 is arranged
below the coupling 37. The spring stopper 51 is secured to the
rotating section of the dresser drive shaft 32, and the lower end
of the spring 50 is secured to the ball spline 35. The spring 50,
the ball spline 35, and the dresser drive shaft 32 are rotated in
unison. In this example also, the difference .DELTA.S (i.e., the
amount of correction) between the pressing force F of the dresser
disk 31 and the measurement value F' of the load cell 45 is
determined according to the same procedures as discussed above.
[0087] FIG. 10 is a schematic view showing the dressing unit
according to a fourth embodiment of the present invention.
Structures and operations of this embodiment, which will not be
described below, are identical to those of the above-described
second embodiment, and repetitive descriptions thereof will be
omitted. As shown in FIG. 10, the dressing unit includes a position
sensor 55 for measuring the position of the dresser disk 31 in the
vertical direction. This position sensor 55 is secured to the
spring stopper 51, so that the position sensor 55 moves in the
vertical direction in unison with the dresser drive shaft 32. The
position sensor 55 has a probe contacting the support base 52 on
which the spring 50 is mounted. The position sensor 55 measures a
relative position of the dresser drive shaft 32 in the vertical
direction with respect to the support base 52, i.e., the position
of the dresser disk 31 in the vertical direction. This position
sensor 55 is a contact-type position sensor whose probe contacts a
measurement target, but a non-contact-type position sensor may be
used alternatively.
[0088] The pressing position of the dresser disk 31 is lowered
according to the wear of the polishing pad 10. Therefore, the
amount of the wear of the polishing pad 10 can be expressed as a
displacement of the pressing position of the dresser disk 31 (i.e.,
displacement from an initial pressing position). Thus, the position
sensor 55 measures the position of the dresser drive shaft 32 in
the vertical direction when the dresser disk 31 is in contact with
the polishing pad 10 to thereby indirectly measure the amount of
the wear of the polishing pad 10. The measurement value of the
position sensor 55 is transmitted to the pressure controller 47,
where the measurement value from the position sensor 55, i.e., the
amount of the wear of the polishing pad 10, is monitored.
[0089] As the polishing pad 10 wears, the lifting force Fb of the
spring 50 is increased. As a result, the pressing force F of the
dresser disk 31 against the polishing pad 10 is decreased. When the
pressing force F is decreased, intended dressing of the polishing
pad 10 may not be performed. To avoid such drawback, the pressure
controller 47 increases the air pressure in the upper pressure
chamber of the air cylinder 36 so as to compensate for the decrease
in the pressing force F. The decrease in the pressing force F is
due to the change in the lifting force Fb of the spring 50 as a
result of the wear of the polishing pad. Therefore, an amount AF of
the decrease in the pressing force F is given by
.DELTA.F=k.DELTA.Z (11)
where .DELTA.Z represents a displacement of the pressing position
of the dresser disk 31, i.e., the amount of the wear of the
polishing pad 10.
[0090] The pressure controller 47 calculates the amount .DELTA.Z of
the wear of the polishing pad 10 from the measurement value
obtained from the position sensor 55, and calculates the amount
.DELTA.F of the decrease in the pressing force in accordance with
the above equation (11). Further, the pressure controller 47
determines the air pressure .DELTA.Pc for generating the obtained
value .DELTA.F using
.DELTA.Pc=.DELTA.F/A (12)
where A represents an effective pressure-receiving area of the
piston 36a.
[0091] The pressure controller 47 increases the air pressure in the
upper pressure chamber of the air cylinder 36 by .DELTA.Pc to
thereby correct the force Fc, generated by the air cylinder 36, in
accordance with the amount of the wear of the polishing pad 10.
This correcting operation enables the dresser disk 31 to dress the
polishing pad 10 at a constant pressing force F, regardless of the
wear of the polishing pad 10.
[0092] FIG. 11 is a schematic view showing a modified example of
the dressing unit according to the fourth embodiment of the present
invention. In this modified example, the spring 50 is arranged
below the coupling 37. The spring stopper 51 is secured to the
rotating section of the dresser drive shaft 32, and the lower end
of the spring 50 is secured to the ball spline 35. The spring 50,
the ball spline 35, and the dresser drive shaft 32 are rotated in
unison. The position sensor 55 is supported by an arm 53 secured to
the non-rotating section of the dresser drive shaft 32. The probe
of the position sensor 55 is in contact with the support base 52.
The amount of the wear of the polishing pad 10 is measured
indirectly by the position sensor 55.
[0093] FIG. 12 is a schematic view showing the dressing unit
according to a fifth embodiment of the present invention.
Structures and operations of this embodiment, which will not be
described below, are identical to those of the above-described
fourth embodiment, and repetitive descriptions thereof will be
omitted. In this embodiment, load cell 45, serving as a
load-measuring device, is provided in the dresser drive shaft 32.
This load cell 45 is located between the air cylinder 36 and the
spring 50 and is coupled to the pressure controller 47 via the
amplifier 46. As with the first embodiment, the pressure controller
47 performs the calibration for determining the relationship
between the pressing force of the dresser disk 31 and the pressure
of the air supplied to the upper pressure chamber of the air
cylinder 36, based on the measurement values obtained from the load
cell 45 and the measurement values obtained from the pressure
sensor 42.
[0094] In the structures shown in FIG. 12, the pressure controller
47 may perform the calibration when the amount of the wear of the
polishing pad 10, which is determined from the measurement value of
the position sensor 55, has reached a preset value. The calibration
according to the amount of the wear of the polishing pad 10 can
prevent a variation in the pressing force F of the dresser disk 31.
Further, the calibration may be performed regularly in
synchronization with pad search which is carried out by the top
ring unit 20 (see FIG. 2). The pad search is an operation of
searching for a reference height of the top ring 21 when polishing
a substrate. More specifically, the top ring 21 is lowered from its
elevated rest position until it contacts the polishing pad 10, and
a height of the top ring 21 when contacting the polishing pad 10 is
determined to be the reference height for the polishing
process.
[0095] In a preferred example, the pressure controller 47 controls
the pressure Pc of the air supplied into the upper pressure chamber
of the air cylinder 36 based on the measurement value of the load
cell 45 such that the dresser disk 31 maintains a predetermined
target pressing force during dressing of the polishing pad 10. Such
feedback control can enable the dresser disk 31 to keep its
pressing force F constant regardless of the wear of the polishing
pad 10.
[0096] FIG. 13 is a schematic view showing a modified example of
the dressing unit according to the fifth embodiment of the present
invention. In this modified example, the spring 50 is arranged
below the coupling 37. The spring stopper 51 is secured to the
rotating section of the dresser drive shaft 32, and the lower end
of the spring 50 is secured to the ball spline 35. The spring 50,
the ball spline 35, and the dresser drive shaft 32 are rotated in
unison. The position sensor 55 is supported by the arm 53 secured
to the non-rotating section of the dresser drive shaft 32. The
probe of the position sensor 55 is in contact with the support base
52. The amount of the wear of the polishing pad 10 is measured
indirectly by the position sensor 55.
[0097] FIG. 14 is a schematic view showing the dressing unit
according to a sixth embodiment of the present invention.
Structures and operations of this embodiment, which will not be
described below, are identical to those of the above-described
third embodiment, and repetitive descriptions thereof will be
omitted. In this embodiment, spring 50 is arranged above the load
cell 45. More specifically, the spring 50 is provided inside the
air cylinder 36 and is arranged so as to press the piston 36a from
below. It is noted that the location of the spring 50 is not
limited to this example and the spring 50 may be located in other
places as long as the spring 50 is arranged between the air
cylinder 36 and the load cell 45.
[0098] In this embodiment, the difference between the actual
pressing force of the dresser disk 31 and the measurement value of
the load cell 45 corresponds to the weight of the dresser assembly.
The difference between the pressing force F of the dresser disk 31
and the measurement value F' of the load cell 45 will be described
below.
[0099] In the initial state (i.e., Fc=0, Z=0, F=0), only the
downward force m.sub.2g acts as a tensile force on the load cell
45. The lifting force Fb of the spring 50 and the weight m.sub.1g
of the upper part of the dresser assembly do not act on the load
cell 45. Therefore, the measurement value F' at the load cell 45 is
expressed as
F'=-m.sub.2g (13)
[0100] When the air is supplied into the upper pressure chamber of
the air cylinder 36 to lower the dresser assembly slightly from the
initial position and the dresser assembly is still suspended in the
air (i.e., Fc.noteq.0, Z.noteq.0, F=0), the following equation
holds from the condition of equilibrium of forces.
F = Fc - Fb + m 1 g + m 2 g = Fc - ( Fb 0 + k Z ) + m 1 g + m 2 g =
0 ( 14 ) ##EQU00005##
[0101] Since the above equation (14) contains the variable Z, the
dresser assembly comes to rest at a certain position that depends
on the force Fc. Therefore, even if the pressing force F of the
dresser disk 31 is zero or approximately zero, the position of the
dresser disk 31 is stable. This indicates that the dresser disk 31
can dress the polishing pad 10 at a very small force.
[0102] In this suspended state, only the downward force m.sub.2g
acts as a tensile force on the load cell 45. Therefore, the
measurement value F' of the load cell 45 is expressed as
F'=-m.sub.2g (15)
[0103] When the dresser disk 31 is in contact with the polishing
pad 10 (i.e., Fc.noteq.0, Z.noteq.0, F.noteq.0), the pressing force
F is expressed as
F=Fc-Fb+m.sub.1g+m.sub.2g (16)
[0104] On the other hand, the measurement value F', which is the
output of the load cell 45, is expressed as
F'=Fc-Fb+m.sub.1g (17)
[0105] Accordingly, the difference .DELTA.S between the pressing
force F and the measurement value F' is given as follows.
.DELTA.S=F-F'=m.sub.2g (18)
[0106] Therefore, the pressing force F can be given by adding the
difference .DELTA.S (=m.sub.2g), as the amount of correction, to
the measurement value F' of the load cell 45. This amount of
correction .DELTA.S can be obtained by a measurement value of the
load cell 45 when the dresser disk 31 is out of contact with the
polishing pad 10. Alternatively, a load cell for calibration may be
placed between the dresser disk 31 and the polishing pad 10 to
obtain an actual pressing force of the dresser disk 31 applied to
the polishing pad 10, and the amount of correction .DELTA.S may be
determined by subtracting the measurement value F' of the load cell
45 from the actual pressing force (i.e., the measurement value of
the load cell for calibration). Since the amount of correction
.DELTA.S (=m.sub.2g) does not contain the variable Z, the value
.DELTA.S is constant regardless of the wear of the polishing pod
10. Therefore, once the amount of correction .DELTA.S is
determined, the value thereof can be used as it is
repetitively.
[0107] As with the first embodiment, the pressure controller 47
performs the calibration for determining the relationship between
the pressing force of the dresser disk 31 and the pressure of the
air supplied to the upper pressure chamber of the air cylinder 36,
based on the measurement values of the load cell 45 and the
measurement values of the pressure sensor 42. This calibration is
performed automatically by the pressure controller 47 at a
predetermined timing, e.g., immediately before or immediately after
dressing of the polishing pad 10. The dressing unit of this
embodiment may include the position sensor 55 according to the
fifth embodiment. In this case, as discussed in the fifth
embodiment, it is preferable that the pressure controller 47
perform the calibration when the amount of the wear of the
polishing pad 10, which is determined from the measurement value of
the position sensor 55, has reached a preset value.
[0108] The previous description of embodiments is provided to
enable a person skilled in the art to make and use the present
invention. Moreover, various modifications to these embodiments
will be readily apparent to those skilled in the art, and the
generic principles and specific examples defined herein may be
applied to other embodiments. Therefore, the present invention is
not intended to be limited to the embodiments described herein but
is to be accorded the widest scope as defined by limitation of the
claims and equivalents.
* * * * *